Trajectory management of metered arrivals into high-density terminal airspace is a critical component for NextGen Trajectory Based Operations concepts. Significant research has focused on the utilization of modern flight management systems (FMS) to enable continuous descent planning, at least from cruise to a metering fix within the Terminal Radar Approach Control Facilities (TRACON) airspace in the United States or the Terminal Control airspace in other countries. Field trials over the past two decades have provided a great deal of insight into the ability to predict and execute such continuous descents for major-carrier type aircraft such as Boeing and Airbus. However, little attention has been paid to “small” (regional, business and light) jet types, which comprise a large and potentially high-growth portion of NextGen traffic operations.
Unlike the larger aircraft types, which are equipped with performance-based FMS systems that attempt to optimize the vertical profile with near-idle descents, the smaller jets are equipped with simpler Vertical Navigation (VNAV) capabilities. Descent planning for these types typically involves a fixed-flight path angle (FPA) descent that is either based on a company-programmed default or a pilot-selected value. For example, the “standard operating procedure” of one large regional carrier called for an indicated airspeed of 320 knots for descent, initiated at the cruise Mach number, using a default FPA of −3.8 degrees. This works fine for nominal conditions with light to moderate winds and no Air Traffic Control interruptions to the descent. However, when speed restrictions are issued by controllers for metering and spacing, the nominal descent plan can become inefficient and difficult, if not sometimes impossible, to fly in strong tailwinds. In addition, random observations of regional jet operations and pilot interviews revealed that a large variety of descent-planning techniques are used by pilots, even for the same equipment. These techniques vary in terms of the selection of descent angle, bottom-of-descent planning, and top-of-descent transition. Sometimes they take into account winds aloft and weight, but rarely descent speed. It is important to develop and standardize the procedures for establishing efficient descent FPAs for small jets. Such standardized procedures lead to better trajectory predictability and provide benefits for separation assurance. However, selecting the FPA is a non-trivial task, as the most fuel-efficient and “flyable” FPAs can vary significantly as a function of aircraft type, weight, speed profile, and, particularly, winds and wind gradient. The systematic effect of these variables on the selection of fuel-efficient and flyable FPAs is far from understood, and only a limited analysis of fixed-FPA descents exists in the literature. Under some conditions, a steep descent may be the most fuel efficient and yet be operationally unacceptable to pilots. Even if a steep descent is achievable with the utilization of speed brakes, many pilots are reluctant, if not unwilling, to use them because of noise and ride discomfort. Given the significant variation in the winds aloft from one area of the National Airspace System (NAS) of the United States to another, and from one day, week or month to another, the present inventors suggest that the FPA procedure may need to be “adaptive”.
In the United States, the Traffic Management Advisor (TMA) computes metering-fix scheduled times of arrival (STA) at the TRACON boundary in order to control throughput of en-route traffic arriving at a high-density airport. The Efficient Descent Advisor (EDA), which was developed to assist en-route controllers in achieving TMA's STAs while maintaining separation, computes clearance advisories that also enable fuel-efficient continuous descent arrivals (CDA). Consider an arrival that is guided by a controller using EDA to plan and execute a continuous descent in order to cross a TRACON metering fix at the STA specified by TMA. During periods of congestion, this STA will result in a small delay at the metering fix to keep the TRACON arrival traffic manageable. Depending on the conditions for each aircraft, speed reductions are typically able to absorb three to four minutes of delay for flights about 20 minutes or 150 nmi from the metering fix. The previous development and testing of EDA focused primarily on descent procedures for large jets equipped with a performance-based FMS. While a simple, fixed-FPA descent procedure using prescribed clearances has been known, EDA itself still lacks a defined descent procedure and corresponding algorithm for defining the descent FPA for small jets.